Lance Cpl. Greg Buckley Jr., 21, was killed Friday, family members said.
The Department of Defense had not made an official public statement by late Sunday, but The Associated Press reported that Buckley and two other Marines were shot by an Afghan ally.
"They were supposed to be protected," the Marine's grief-stricken father, Greg Buckley Sr., said outside his Oceanside home Sunday afternoon. He was joined by his son's friends, dozens of relatives and neighbors who stopped by to pay their respects, bringing food, flowers and photographs.
The elder Buckley, his ex-wife Marina Buckley and their two surviving sons will fly to Dover, Del., where mortuary operations are conducted at Dover Air Force Base, and then return Tuesday with his remains. The family has not yet made funeral plans.
Buckley turned 21 two weeks ago. He graduated in 2009 from Oceanside High School, where he played on the varsity basketball team.
His family said Buckley was to return stateside this week, and he planned to surprise friends in Miami Beach, Fla.
Buckley wrote to his family in June, saying he had grown increasingly homesick while in Afghanistan. In the letter he wrote, "I hate it here" but told his family how much he appreciated them. "I love all of you and I think about all four of you every day," he wrote.
And in phone conversations, he would count down the days until his return stateside. " 'I've got 20 days. Dad, 15 days, 11 days,' " his father recalled.
"I can't express how much I miss him. He was my world," said his mother, Marina Buckley, 48, of Oceanside. When she spoke with him last week via the Skype video service, "he seemed great," she said.
Buckley's brothers -- Shane, 18, and Justin, 16 -- called him a role model. "He meant the world to me," Shane Buckley said. "He was just so caring . . . he'd always come into my room at night and just talk to us about anything we needed to talk about, from girls to sports."
In his letter home, Buckley told his brothers, "I want you guys to really enjoy your summer, but at the same time be smart."
Sebastian Buttafuoco, 21, a friend of the Marine since they were 5, said he felt like a part of him was missing. "We had so many plans," he said. "He was going to be the best man at my wedding, the godfather of my children -- now it's all gone."
Buckley's father said his son announced his intention to join the Marines when he was 16, remembering the 9/11 attacks.
"I was dead against it," his father said. But his son enlisted anyway, during the spring of his senior year.
In high school, Buckley "always made everyone laugh," Buttafuoco said. "Whatever he could do to make someone smile, he did."